1. Field of the Invention
This invention relates generally to a tubing centralizer, and more particularly, but not by way of limitation, to a tubing centralizer having a double spring cage with a close tolerance with the inside diameter of a tubing casing.
2. Description of the Related Art
When a subterranean well is drilled, the wellbore is often “cased” or lined with steel pipe called casing to keep the formation from caving in and filling the wellbore. When casing is installed across the producing formation, the desired casing interval will often be perforated to allow the produced fluids to enter the wellbore to be produced to the surface.
A tubing string of smaller diameter is then installed in the cased wellbore and “hung” in the wellhead at the surface. A joint of tubing is typically 31.5′ in length and is joined together in end to end fashion utilizing tubing couplings. The top end of the tubing string is attached in the wellhead and the bottom end can be attached to a downhole tubing anchor catcher (“TAC”). The tubing string is a downhole pipeline between the downhole pump and the wellhead. The tubing string helps transport the produced fluids from the producing reservoir through perforations in the casing up to the surface and subsequently into storage and sales tanks.
The rodstring and downhole pump is then installed within the tubing string. The downhole pump is installed in the pump seating nipple located in the bottom of the tubing string. The rodstring is the connection between the downhole pump and the pumping unit at the surface. The reciprocating motion of the horse head of the pumping unit and the connected rodstring actuates the downhole pump, lifting produced fluids up the tubing within the space between the tubing string and the rodstring.
If the tubing string is installed without a tubing anchor catcher, the reciprocating movement of the rodstring inside the tubing string may cause the tubing string to move with the reciprocating movement of the rodstring. This movement of the tubing string can reduce the length of the downhole pump stroke reducing the volume of produce fluids the downhole pump can lift to the surface through the tubing string. This movement of the tubing can contribute to the following downhole issues:                1. The exterior wall of the tubing can contact the interior wall of the casing and wear a hole in the casing, resulting in a casing leak. This casing leak allows formation fluids to enter the producing wellbore, damaging all components of the downhole lift system.        2. The exterior wall of the tubing can make contact with the interior wall of the casing and wear a hole in the tubing string, resulting in a tubing leak. This “exterior” tubing leak allows fluids from the tubing to flow into the tubing-casing annulus, reducing the lift capability of the downhole pump.        3. The rodstring can make contact with the interior wall of the tubing and wear a hole in the tubing string, resulting in a tubing leak. This “interior” tubing leak allows fluids from the tubing to flow into the tubing-casing annulus, reducing the lift capacity of the downhole pump.        4. The rodstring can make contact with the interior wall of the tubing and wear the outside surface diameter of the rodstring until the rodstring can no longer support the fluid load produced by the downhole pump. The rodstring then separates, resulting in a rod failure. A rod failure stops the rodstring from lifting fluids from the pump to the surface.        
Tubing anchor catchers can be installed in wells to reduce the above-mentioned tubing movement of the tubing string during producing operations. Tubing anchor catchers are installed in the tubing string and actuated to secure the tubing anchor catcher to the interior casing wall. Once the tubing anchor catcher is set, the tubing string is stretched to place the tubing string between the tubing anchor catcher and the wellhead in tension. This tension should eliminate all tubing movement of the tubing between the tubing anchor catcher and the wellhead.
Eliminating this tubing movement should reduce the frequency of the four (4) downhole failures mentioned above.
When the tubing anchor catcher is located above the pump, there can still be movement of the tubing string between the location of the tubing anchor catcher and the downhole pump. This remaining tubing string movement below the tubing anchor catcher may still produce possible casing leaks, tubing leaks, rod failures, and reduced production of wellbore fluids to surface.
Operators prefer to install tubing anchor catchers as close above the top perforation of the top producing interval as possible to avoid formation solids from entering the wellbore, settling on top of the tubing anchor catcher, and sticking or “planting” the tubing anchor catcher in the casing.
When the producing interval (the distance between the top and bottom perforation) in a producing well is short, the tubing anchor catcher is usually installed above the top perforation and the pump is installed near the bottom perforation. Installing the downhole pump below the perforations also allows formation gas to produce into the tubing casing annulus instead of moving through the pump, causing gas interference and reducing the efficiency of the pump to lift fluids up to the surface.
A short distance between the pump and the tubing anchor catcher minimizes tubing movement, which can cause casing leaks, tubing leaks, and rod failures and increases the lift capability of the downhole pump.
When the producing interval is longer because of (1) a large single producing zone, (2) multiple producing zones resulting in a large producing interval, (3) the addition of new producing zones, or (4) horizontal or deviated wellbores, the tubing anchor catcher can also be installed above the top perforation and the pump installed near the bottom perforation. This longer un-anchored tubing section can still cause reduced lift efficiency of the downhole pump, resulting in reduced volumes of wellbore fluids, increased tubing movement between tubing anchor catcher and downhole pump, and the increased possibility of casing leaks, tubing leaks, and rod failures.
Based on the foregoing, it is desirable to provide a tubing centralizer that can be installed between the tubing anchor catcher and the seating nipple of the downhole pump in a tubing string to help centralize the tubing string within the casing string and reduce movement of the tubing string. This would reduce tubing-on-casing contact and rod-on-tubing contact, reduce casing leaks, reduce tubing leaks, and reduce rod failures. Furthermore, reducing the length of unanchored tubing will help maximize the volume of produced wellbore fluids.